Change-Id: If7d6e58b19f98ccb7cc4c209e458cb6f4f4765ad

EPEL is no longer required for a default installation. Let's disable it.

Also clean up the install_epel variable from Kolla Ansible globals.yml
template, since it never existed.

Story: 2009757
Task: 44227

Change-Id: I96eb4685f997e85ad2ee5318640d58d0287a016d

The variables in yum.yml and yum-cron.yml were deprecated in Ussuri.
This patch removes them, and updates the defaults in dnf.yml.

Story: 2008160
Task: 40906

Change-Id: I97cc98dd2ff726e5885fefcab17f17796d9fd453

Adds support for configuration of DNF repo mirrors for CentOS and EPEL
repositories, as well as custom repositories.

Adds support for DNF automatic, which is a replacement for yum-cron.

Configuration is backwards compatible, falling back to the equivalent
yum variables when DNF variables have not been overridden.

Change-Id: I8bef5e9c8e1c77c25d6077ff690da8f2cde6a643
Story: 2006574
Task: 38922